It works.
1. Install icloud for Windows. Restart computer.
2. Launch icloud for Windows, select to sync bookmarks. Tick both IE and Firefox. When the download firefox option comes up download and install for Firefox.
3. Accept the one or two requests the extension asks for regarding permissions. Extension will show it's now connected to icloud.
4. Go back to icloud for Windows and hit save. Firefox will no longer be ticked, just IE, but it'll continue to sync in the background, so long as bookmark syncing is turned on (with IE) and the firefox extension remains installed.